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A051948
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Numbers k such that k*sigma(k) == 2 (mod phi(k)).
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1
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2, 3, 4, 5, 6, 7, 11, 13, 17, 19, 22, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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REFERENCES
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R. K. Guy, Unsolved Problems Number Theory, B37.
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LINKS
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Chris Nash, There no other solutions besides 4, 6, 22 and the primes (html, pdf)
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MATHEMATICA
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Select[Range[2, 250], Mod[#*DivisorSigma[1, #]-2, EulerPhi[#]]==0&] (* Harvey P. Dale, Sep 22 2021 *)
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PROG
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(PARI) a(n)=if(n<12, [2, 3, 4, 5, 6, 7, 11, 13, 17, 19, 22][n], prime(n-3)) \\ Charles R Greathouse IV, Aug 26 2011
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CROSSREFS
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KEYWORD
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nonn,nice,easy
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AUTHOR
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STATUS
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approved
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